When carrying out processes such as chemical or biochemical analyses, assays, syntheses, or preparations, a large number of separate manipulations are performed on the materials being processed. These manipulations include the measuring, aliquotting, transferring, diluting, mixing, separating, detecting, and incubating of the materials. Microfluidic technology miniaturizes these manipulations and integrates them so that they can be executed within a single microfluidic device. For example, pioneering microfluidic methods of performing biological assays in microfluidic systems have been developed, such as those described in U.S. Pat. No. 5,942,443 entitled “High Throughput Screening Assay Systems in Microscale Fluidic Devices” by Parce et al. and in PCT Published Application Number WO 98/45481 entitled “Closed Loop Biochemical Analyzers” by Knapp et al.
A problem of particular interest in numerous applications of microfluidic devices is the detection, characterization, and quantification of reactions that cannot be conveniently monitored by taking fluorogenic measurements. Such reactions include reactions in which there is no measurable change in fluorescence when a reaction product is formed. For example, kinase reactions have no easy fluorogenic means of quantification. Mobility shift assays in microfluidic devices were devised to help overcome this problem. Mobility shift assays are described in U.S. Pat. No. 6,524,790 entitled “Apparatus and Methods for Correcting for Variable Velocity in Microfluidic Systems,” by Kopf-Sill et al.
The mobility shift assays currently carried out in microfluidic devices could be improved by increasing the throughput of those assays, and by expanding the applicability of those assays to reactions not compatible with existing mobility shift assays. The throughputs of some existing mobility shift assays are adversely affected by the long transit times required to separate some molecules based upon their electrophoretic mobility. These long transit times can lead to increased thermal dispersion of the separated groups of molecules, as well as hydrodynamically induced dispersion when pressure driven flow is used. Dispersion can adversely affect the throughput rate of an assay and decrease separation resolution. Accordingly, minimizing the transit times in a mobility shift assay can increase the throughput and improve the resolution of an assay.
A welcome addition to the art would be the enhanced ability to increase the throughput and resolution of mobility shift assays by decreasing the transit time. The present invention describes and provides these and other features by providing new methods and microfluidic devices that meet these and other goals.